Calibration of an output parameter of an analog circuit typically depends on adjustment of one or more adjustable components within that circuit. For example, voltage references are a typical precision analog circuit, where adjustment may be required.
In some cases, the adjustable component is a resistor, and the output parameter of the circuit may be directly adjusted by trimming the resistance value of the resistor. However, there are also many cases of electric circuits where the dependence of the output parameter on the resistor may be indirect. For example, the temperature variation of the output parameter may be affected by the resistance value. In most cases, the behavior of certain resistors in a circuit will have an impact, whether direct or indirect, on the behavior of the output parameter of the entire circuit.
Voltage reference circuits are used in larger electric circuits in cases where a known and stable reference voltage is desired, from which to derive other voltages in the larger circuit. For example, 16-bit absolute resolution corresponds to roughly 15-ppm precision in a reference voltage. The precision and accuracy of the voltage reference are of paramount importance in the usefulness of a voltage reference in an electric circuit. This fact is manifest in the price-performance strata in the market for voltage references.
Schematics of typical voltage reference circuits from the prior art are shown in FIGS. 20 and 21. In most cases, there are a plurality of resistors, whose purpose is generally to adjust the output voltage in the desired output range, and adjust the coefficients of temperature variation of the output voltage to be preferably close to zero.
Devices, such as voltage references, made by modern CMOS or BICMOS process technologies suffer from unavoidable statistical spreads in device parameters, such as output voltage and temperature coefficients. If high-precision analog performance is desired, some form of high-precision calibration or trimming is therefore essential.
The resistors involved in a voltage reference may potentially be trimmed, for example by using laser trimming, Zener zapping, external manual trimpot, or other resistance trimming methods. Laser trimming can be done only before packaging, and has limited precision. Furthermore, during or after packaging, trimmed parameters can shift away from the required values. Zener zapping can be done after packaging, but has quite limited precision due to the on-chip area required for a configurable array of fixed resistors. Potential disadvantages of manual trimpots are their manual operation, and their relative temperature coefficient of resistance (RTCR) may be non-zero, which may degrade the temperature behaviour of the overall voltage reference.
One of the most popular physical voltage reference cell is the “bandgap cell”, depicted in FIG. 20, which uses the base-emitter voltage (Vbe) of a bipolar transistor to produce approximately 1.2V at −273° C. (0 K). This cell was the subject of a patent (U.S. Pat. No. 3,887,863). In order to serve as reference for other practical circuit voltage levels such as 2.0, 2.048, 2.5, 4.096, 5.0, 7.5 and 10.0V, an amplifier is typically involved, with an amplification factor, defined by a pair of resistors (included in the amplifier depicted in FIG. 20). This amplification requires a precisely-trimmed and predictable ratio of those resistors internal to the amplifier. For temperature stability of the overall voltage reference, the relative temperature coefficient of resistance (RTCR) of this pair of resistors is quite important. One or more other pairs of resistors, such as the pairs R3,R4 and R1,R2, and are also needed in the circuit, in order to compensate for the temperature dependence of Vbe, which is approximately −2 mV/K. These other pairs of resistors must also be accurately tuned, to high precision, and their RTCR is also important. Another patent, (U.S. Pat. No. 4,250,445), and patent application (US#2003/0006831A1), teach that it is advantageous to be able to modulate the RTCR of such pairs of resistors. These pairs of resistors are commonly included on-chip, along with the physical cell and the amplifier, in commercial standard voltage reference integrated circuits.
Further pair or pairs of resistors, internal or external to the integrated chip, are also typically used with higher-quality voltage references, to fine-adjust the output reference voltage. An example of such a pair of resistors is shown as R5/R6 in FIG. 20. Many standard voltage references come with standard external pins for “trim”, in order to allow the user to fine-adjust the reference voltage in a circuit using an external voltage divider.
This final adjustment could also be important even if the precision of the voltage reference is already sufficient for the desired application in the case where the output voltage needs to be trimmed away from an initial value. An example is in applications involving binary scaling (where one might need 10.24V Instead of 10.00V, for example), or to compensate for the voltage drop across the conductors on a printed-circuit board. In such cases the user would be responsible for the RTCR of the external voltage divider, and its influence on the overall temperature dependence. Typically the user may select a manual trimpot for this purpose. Note that, if the precision and/or temperature characteristics of this external trimpot are not matched to the quality of the voltage reference chip, this can substantially degrade the overall accuracy and stability of the output reference voltage. So, for high-end industrial trimpots, the user may typically use a high-quality multi-turn manual trimpot.
An approach similar to that outlined for the bandgap reference above is used with other types of physical reference cells, such as “Zener” reference cells shown in FIG. 21 (U.S. Pat. No. 5,252,908). In general, many voltage reference integrated circuits have a network of internal resistors, and often a provision for an external voltage divider. High-precision control of both the resistance values and RTCR is important in all of these resistances.
In the fine tuning of a voltage reference, it is typically relevant to consider voltage adjustment to precision in the range of a few ppm. Initial accuracy (without an externally-trimmed voltage divider) in the range of +/−200 ppm (˜12 bit resolution) to +/−2000 ppm (˜8 bit resolution) is typical of modern good-quality voltage references. Temperature coefficients of less than 1 ppm/K are typical for the best-quality voltage references.
Top-quality commercially-available industrial voltage references typically have +/−100 ppm absolute voltage accuracy. Voltage references based on the bandgap cell (FIG. 20) typically do not achieve this performance. Zener-based voltage references, similar to that depicted in FIG. 21, may achieve this performance but require substantially more power, tending to make Zener-based high-precision voltage references unsuitable for battery-powered applications.
Therefore, there is a need for a circuit and method which allows adjusting output parameters of circuits to a higher precision than the techniques of the state of the art.